PROJECT SUMMARY This project seeks to surmount current limitations in our understanding of early infant immunity through longitudinal genomic and cellular studies of immune development and primary responses to routine two-month vaccines. Infants and young children are more susceptible to invasive infections than adults owing to overall reduced competency of protective immune responses, including to vaccines, which require administration of multiple doses over several months for adequate long-term protection. While immunization programs have dramatically decreased the global morbidity and mortality caused by infections, it remains that infectious diseases are the most frequent cause of death in infants and young children. The cellular, molecular and genomic mechanisms that contribute to this vulnerability are largely unknown. Ever more powerful tools in genomics and systems biology offer exciting opportunities to resolve these knowledge gaps through detailed analysis of the transcriptomic, epigenomic and functional signatures of infant immune cell populations. However, such studies have been limited by the difficulty in accessing clinical samples from infants, the incompatibility of many genomic technologies for use in small-volume samples, and the lack of bioinformatic tools for integrating and interpreting complimentary yet complex datasets. This proposal will capitalize on our experience studying the infant immune response, our access to infant populations, and our expertise in developing immunogenomic assays for use in human blood-derived immune cells (PBMCs). Specifically, we propose a longitudinal analysis of PBMCs from infants i) at 2, 6 and 12 months, to establish the baseline cellular, phenotypic and genomic signatures of immune development (Aim 1), and ii) at key time points over the course of routine two-month vaccinations, to identify the cellular, phenotypic and genomic signatures associated with primary immune responses to vaccines (Aim 2). We will use an innovative immunogenomic Profiling and Analysis Pipeline (iPAP) we developed that allows us to extract maximal transcriptomic (RNA-seq), epigenomic (ATAC-seq), isoformic (SMRT-seq), cytometric (50-parameter flow cytometry) and immunophenotypic (CyTOF) information from a single infant blood sample, and to integrate these distinct datasets for unparalleled depth of insight into the correlated cellular and genomic signatures of immune development and vaccine responsiveness. Our approach is unbiased, multifaceted and highly technology-driven, combining many of the most cutting-edge genomic and quantitative cell-based technologies with our deep experience in applying these technologies for use in human infant immune cells. In line with the goals of this RFA, this project will yield a comprehensive dataset from infants that can be used to identify fundamental mechanisms and pathways associated with immune development and primary responses to vaccines, and will set the stage for future studies aimed at designing new interventions that induce more potent and protective immune responses for young infants.